In "Good Country People," how does O'Connor use situational irony to show the motivations of the characters?

Situational irony is a literary device in which the outcome of a character's actions or circumstances is the opposite of what is expected. In "Good Country People" by Flannery O'Connor, this tool is used to reveal the ulterior motives of the central characters—Hulga, a doctor of philosophy with a wooden leg, and Manley Pointer, a Bible salesman who encounters Hulga when he stops at her family's home.
As a result of their initial conversation, Manley invites Hulga to go on a picnic the next day. Hulga is excited by the possibility of a relationship, but she also seeks to manipulate him both intellectually and sexually. For example, she imagines a scenario in which "she very easily seduced him and that then, of course, she had to reckon with his remorse. True genius can get an idea across even to an inferior mind." Clearly, Hulga sees Manley's simple country persona and adherence to the Bible as "inferior" to her scholarly expertise in philosophy. Manley, therefore, is an opportunity for a relationship as well as a possible subject of control.
However, the date does not turn out as Hulga expects it will—in fact, Manley himself is not who she thinks he is. Manley is actually a predator who seduces women with physical handicaps and steals their prosthetic body parts. Instead of Hulga seducing Manley, he instead convinces her to take off her wooden leg, then puts it in his suitcase and leaves her abandoned in a hayloft. In contrast to Hulga's expectations, he even tells her that she "ain't so smart. I been believing in nothing since the day I was born."
One use of situational irony here is that while Hulga's motivation is to manipulate Manley, she is the one who is hoodwinked by him in the end. Also, Manley as a character turns out to be different from both Hulga and the reader's expectations, as his motivation is shown to be to seduce Hulga into giving him her leg. The surprise outcome of the story therefore demonstrates what each of the characters wants and the drastic difference between what Hulga hopes will happen and the events that actually take place.
http://typesofirony.com/situational-irony/